The development of informatization has called attention to the development of an electronic paper display which is thin and lightweight and is an alternative to paper, an IC tag which enables each product to be identified instantly, or the like. In addition, liquid crystal display devices are widely used for display devices ranging from large-sized display devices such as television sets to small-sized display devices such as mobile phones. As these devices widely spread, development has been conducted to reduce cost and add higher values. In particular, in recent years, the global environment has received increasing attention, and the development of a device capable of operating with less power consumption and at higher speed has been attracting attention.
In these devices, transistors are currently used as elements. A transistor is an element in which regions called a source and a drain are provided in a semiconductor and connected to respective electrodes, potentials are supplied to the electrodes, and an electric field is applied to the semiconductor with the use of an electrode called a gate through an insulating layer or a Schottky barrier so that the state of the semiconductor is controlled, whereby current flowing between the source electrode and the drain electrode is controlled.
In order to realize a device capable of operating with less power consumption and at higher speed, a transistor having a higher on/off ratio and a smaller parasitic capacitance is needed. The on/off ratio refers to the ratio of on-state current to off-state current (ION/IOFF), and the higher the on/off ratio is, the better the switching characteristics are. Note that the on-state current is a current which flows between a source electrode and a drain electrode when a transistor is turned on, and the off-state current is a current which flows between a source electrode and a drain electrode when a transistor is turned off. For example, in the case of an n-channel transistor, the off-state current is a current which flows between a source electrode and a drain electrode when gate voltage is lower than threshold voltage of the transistor. The parasitic capacitance is a capacitance generated in an overlap portion between a source electrode (a drain electrode) and a gate electrode, and an increase in parasitic capacitance leads to an increase in switching time or a decrease in transfer gain for AC signals.
The parasitic capacitance of a transistor varies depending on the area of an overlap portion between a source electrode (a drain electrode) and a gate electrode. By decreasing this area, the parasitic capacitance can be reduced. However, there is a trade-off between the decrease in area and manufacturing cost, and it is very difficult to balance the two.
The on-state current of a transistor varies depending on the length and width of a channel formation region. The length of a channel formation region corresponds to the interval between an edge of a source electrode and an edge of a drain electrode which face each other. By decreasing this length, the on-state current can be increased. The width of the channel formation region corresponds to the length along which the source electrode and the drain electrode face each other. By increasing this width, the on-state current can be increased. For example, a transistor structure is disclosed (see, for example, Patent Document 1) in which a source electrode and a drain electrode of a transistor each have a comb shape and interdigitate with each other so that the transistor can have a channel formation region with a larger width and can be tolerant of a misalignment between the source electrode (the drain electrode) and a gate electrode. However, in such a structure, the area of the overlap between the source electrode (the drain electrode) and the gate electrode is large, which causes an increase in parasitic capacitance. If the area of the overlap between the source electrode (the drain electrode) and the gate electrode is decreased in order to reduce the parasitic capacitance, the on-state current is decreased.
Furthermore, an excessive increase in length of the channel formation region causes the problem of a drastic decrease in the on-state current. In order to maintain the on-state current at a certain value or more, it is important to maintain at a certain value or less the length of the channel formation region (which may alternatively be the length of the overlap portion between the source (drain) electrode and the gate electrode, when the gate width is constant). In other words, a transistor which allows the area of the overlap portion to be decreased without changing the length of the channel formation region (without changing the length of the overlap portion between the source (drain) electrode and the gate electrode, when the gate width is constant) is necessary.